A jungle inside a cave? A roof collapse long ago in Hang Son Doong let in light; plants thickly followed. As "Sweeny" Sewell climbs to the surface, hikers struggle through the wryly named Garden of Edam.

Mist sweeps past the hills of Phong Nha-Ke Bang National Park, its 330 square miles set aside in 2001 to protect one of Asia's largest cave systems. During the Vietnam War, North Vietnamese soldiers hid in caves from U.S. air strikes. Bomb craters now serve as fishponds.

Going underground, expedition members enter Hang En, a cave tunneled out by the Rao Thuong River. Dwindling to a series of ponds during the dry months, the river can rise almost 300 feet during the flood season, covering the rocks where cavers stand.

Headroom shrinks in the middle of Hang En as cavers pass beneath a ceiling scalloped by eons of floodwater rushing past. The river shortly reemerges onto the surface, then burrows into Hang Son Doong after a few miles.

Moss-slick boulders and a 30-foot drop test author Mark Jenkins at the forest-shrouded entrance to Hang Son Doong. "Even though these caves are huge, they're practically invisible until you're right in front of them," Jenkins says. Hunters have found caves by spotting winds gusting from underground openings.

Hang Son Doong's airy chambers sprout life where light enters from above—a different world from the bare, cramped, pitch-black spaces familiar to most cavers. Ferns and other greenery colonize rimstone. In the jungles directly beneath roof openings, explorers have seen monkeys, snakes, and birds.

Rare cave pearls fill dried-out terrace pools near the Garden of Edam in Hang Son Doong. This unusually large collection of stone spheres formed drip by drip over the centuries as calcite crystals left behind by water layered themselves around grains of sand, enlarging over time.

The trickiest challenge for the expedition team was to find a way over the Great Wall of Vietnam, an overhanging mass of flowstone that blocked the way deep inside Hang Son Doong. Climbing specialists "Sweeny" Sewell and Howard Clarke here work on anchoring bolts to the slippery, porous rock to support the weight of climbers using ropes. Once over the wall, the expedition team discovered a second entrance into the cave.

Dubbed the Great Wall of Vietnam, a 200-foot cliff halted the advance of the first team to enter Hang Son Doong, in 2009. When explorers returned, Sewell drilled bolts for climbers to scale the obstacle with ropes. A white streak below, to his right, marks how high water rises during the wet season.

"It sounded like a roaring train," said "Sweeny" Sewell, describing the noise a second before a waterfall exploded into Hang Son Doong through the Watch Out for Dinosaurs doline, or sinkhole opening. A rare dry-season downpour produced the thundering runoff. Were the cavers scared of drowning? "Maybe if it were a smaller cave," said expedition leader Howard Limbert, "but not here."

In the dry season, from November to April, a caver can safely explore Hang Ken, with its shallow pools. Come the monsoon, the underground river swells and floods the passages, making the cave impassable.

Taking the only way in, a climber descends 225 feet by rope into Hang Loong Con. A survey party discovered the cave in 2010, hoping it would connect with the enormous Hang Son Doong. A wall of boulders soon blocked the way, but a powerful draft indicated that a large cavern lay on the other side.